The majority of MEPs were elected on Sunday 7 June, but because of traditional polling days varying from country to country according to local custom, some countries held their elections in the three preceding days:

This was the first European Parliament election that Bulgaria and Romania participated in at the same time as the other member states. When they joined the EU in 2007, they held elections for MEPs outside the normal electoral calendar.

A poll sampling nineteen national polls predicted the following results: EPP 265 MEPs, PES 195 MEPs, ALDE 95 MEPs, GUE–NGL 40 MEPs, Greens-EFA 35 MEPs, UEN 35 MEPs. The remaining 70 MEPs were not predicted, but about 20 were expected to be gained by far-right or Libertas candidates. IND/DEM was predicted to fade away due to Libertas' expected success.[9]

A prediction by political scientists Simon Hix (London School of Economics), Michael Marsh (Trinity College Dublin) and Nick Vivyan (London School of Economics) foresaw little change in the distribution of seats, predicting 249 seats for the EPP, 209 for the PES, 87 for ALDE, 58 for UEN or the European Conservatives, 48 for GUE–NGL, 39 for Greens–EFA, 17 for IND/DEM (resulting in its dissolution) and 29 NI; Libertas was not expected to win any seats.[10][11] A later prediction saw 262 seats for the EPP, 194 for the PES, 85 for ALDE, 53 for UEN or the European Conservatives, 40 for GUE–NGL, 50 for Greens–EFA, 23 for IND/DEM (resulting in its dissolution) and 29 NI.[12]

At the previous election member states were granted a fixed allocation of seats for election as determined under the provisions of the Treaty of Nice (current allocation is in the 2007 column below). The admission of Bulgaria and Romania midway through the previous Parliament's term had increased the overall size of the assembly to 785, and under the terms of the Treaty of Nice it was mandated that the seat allocations be modified for this election, dropping 49 seats to keep the overall size of the Parliament down.

It had been the stated desire of the member-state governments to ratify the Treaty of Lisbon before the election so that its articles governing the European Parliament could enter force as of this election. However, this was blocked by the Irish rejection of the treaty in a referendum. Under Lisbon, there would be a somewhat different allocation of seats, with fewer seats eliminated, leading to a slightly different distribution totalling 751 MEPs. If the Lisbon Treaty is subsequently ratified, it is planned to give the additional seats to the "increasing" countries already before the next elections, bringing the number of MEPs to 754 for a transitional period until 2014.[13]

To have the additional MEPs ready at hand, 18 "phantom MEPs" were supposed to be elected at the election (which, however, did not happen in France). These were expected to get observer status in the European Parliament and full payment from the day the Lisbon Treaty enters into force (which did not happen either), but only become full members of parliament after an additional protocol is ratified.[14][15] The latter will require a decision by the European Council.[16]

These divide into EU-wide and nationally based campaigns, often by non-governmental organisations focussed on specific policy areas. They are designed to influence MEP candidates, those with a strong interest in the issue, and voters in general. Examples of nationally based campaigns include those of the Royal College of Nursing[23][24] and British Overseas NGOs for Development.[25][26]

From 1 April Parliament started putting up election posters, internet banners and billboards encouraging people to vote, with messages such as "How much should we tame financial markets?" to demonstrate the role of Parliament. The same 10 posters, designed by Berlin-based advertising company Scholz & Friends Group, were translated into all 23 languages and have been deployed across the whole of the EU. However, some posters are put up more in countries where the issues they present are of more significance and the most posters will be put up in those countries which had the lowest turnout in 2004. The end slogan is "Use your vote in the European Parliament Elections" with the date of the election in that country. The entire campaign costs 18 million euros, or 0.05 euros per eligible voter, with Germany, Spain and Italy donating some advertising space free of charge.[29]

It had initially been foreseen that the Treaty of Lisbon would have entered into force by the time of these elections, making them the first to be held under its provisions. However, primarily because of the No vote in the referendum in Ireland, the framework established by the Treaty of Nice will be used again. Amongst other differences, the number of MEPs to be returned depends upon which rules are in effect: while 736 MEPs will be elected under the Nice rules, this number would have increased to 751 if the Lisbon Treaty were in force.

A further change that Lisbon would have brought was an increase to the powers of Parliament, including powers over the appointment of the President of the European Commission.[30] It had been suggested by some that political parties could run with candidates for the Commission President;[31] with leaders now linking the post to elections and that convention having been enshrined in the Constitution, further encouraging the possibility.[32]

In 2007, it emerged that current Commission President José Manuel Barroso would likely seek a second term,[19] which he confirmed on 19 July 2008,[33] and he received the backing of French PresidentNicolas Sarkozy.[34] In beginning to formalise the emerging party-political nature of the office, in early 2009 the centre-right People's Party backed Barroso as their candidate and the centre-left socialists opposed, however they did not put forward their own formal candidate.[35]

NB: "Adjusted" figures restate group numbers to proportionately reflect the reduction of the Parliament's size by 49, and to reflect the following political group movements announced prior to the election: DIKO (CY) from ALDE to Other; ODS (CZ) from EPP to Other; FF (IE) from UEN to ALDE; AN (IT) from UEN to EPP; PD (IT) from PES and ALDE to Other; PiS (PL) from UEN to Other; and Conservatives/UCUNF (UK) from EPP to Other.Note:Postal votes missing from Slovenia. Complete results were supposed to be announced on 15 June

18 additional seats were designated to be filled by "observers" when the Lisbon treaty goes into force, to account for new seat distribution with the Lisbon Treaty. On 23 June an intergovernmental conference (IGC) of EU member states amended a protocol attached to the Lisbon treaty on transitional arrangements. Those transitional arrangements were originally envisaged as taking the total number of MEPs from 785 to 751 for the 2009 elections. The amended text says that the protocol “shall enter into force if possible on 1 December 2010”. Germany is allowed to retain its three additional MEPs until the next elections in 2014.

The member states entitled to observer seats were supposed to make provisions before the 2009 elections for who would get the additional seats in the European Parliament.[83][84] However, this was not done by France. This created a limbo for the observers elected from other countries, since all observers are supposed to start their work at the same time. In December 2011, they took their seats.

Traditionally, realignment between the different political groups and European political parties occurs in the runup to the election and in the time between the election and the first sitting of the European Parliament, when the political groups are constituted; the first sitting of the EP elected in 2009 is set for 14 July 2009.[86]

New rules regarding the minimum number of members needed for a political group will come into effect following the 2009 election. The PES and EPP–ED had initially proposed to raise the limit for groups in the EP from 20 MEPs and one fifth of member states to 30 MEPs and a quarter of member states, which would effectively close down UEN (only MEPs from six countries) and I/D (only 22 MEPs). All five smaller groups (UEN and I/D plus ALDE, Greens–EFA and GUE–NGL) protested against these proposed changes.[87] The proposal was voted down 15 to 14 in committee, and in an eventual compromise, new rules were approved in plenary on 9 July 2008 which require 25 MEPs from at least a quarter of the member states (i.e. seven member states).[88] Although UEN and I/D still fail to meet the basic requirements of the new rules, the adopted compromise also included a provision allowing the continued existence of a group which had fallen below the threshold, but only if it still had members from at least a fifth of the member states and only if it had been founded at least a year before dropping below the threshold; if there is "sufficient evidence" of the abuse of this provision, it need not be applied.[89]

The largest single contributor of MEPs to the UEN group, the Italian National Alliance, merged with Silvio Berlusconi's Forza Italia party to form the People of Freedom (PdL) in 29 March 2009.[90] The combined party belonged to the EPP and the MEPs elected in 2009 sat in that parliamentary grouping.[91] This move came after a series of attempts by National Alliance to abandon more staunchly national conservatism and Eurosceptic parties and move towards the moderate European centre-right, including a previous application for EPP membership.

Ireland's Fianna Fáil joined the European Liberal Democrat and Reform Party (ELDR) in 16 April 2009.[92] Fianna Fáil has long been a proponent of European integration and was openly uncomfortable about its European affiliation, with former leader Bertie Ahern often distancing his party from the right-wing positions of his groupmates and describing UEN as purely a "technical arrangement." Fianna Fáil very nearly joined ALDE during the group reshuffle after the 2004 European elections, and had also previously explored seeking membership in ELDR.[93][94]

The Italian Democratic Party was founded in 2007 from two parties in the Olive coalition decided to merge: Democrats of the Left and Democracy is Freedom – The Daisy a socialist and a liberal party respectively. The merged resulted in disagreements over which European political party the Democratic Party would belong to, with many former members of the Daisy party objection to member of the Party of European Socialists (PES).[95] Eventually it was agreed that the party would sit with the PES in the European Parliament but not join the party. The new parliamentary grouping was originally going to be called the "Alliance of Socialists and Democrats for Europe",[96] but this name was too similar to the name of the liberal group, the Alliance of Liberals and Democrats for Europe, so the name was changed to Progressive Alliance of Socialists and Democrats (S&D).[97]

The two largest groups in Parliament, the EPP-ED and PES, agreed to share the post of President of the European Parliament as they have done for all but two terms of Parliament. The first half of the term was taken by Jerzy Buzek MEP of the EPP, who was the first MEP from Eastern Europe to hold the post. He passed it over to Martin Schulz MEP, the current leader of PES.[102] The deal got support form The ALDE group (their candidate Graham Watson dropped out from the race).[103]

The other official candidate was Eva-Britt Svensson, nominated from EUL-NGL.[104] She campaigned with the slogan "a different voice". She was the first MEP from Sweden to candidate to the position.